Copying a cell in Excel spikes CPU to over 50%

Sharing this because it took a long time to figure out and I couldn't find this specific cause documented anywhere.

The issue: Copying any cell in certain Excel files would spike the CPU to 50%+ for half a minute. Didn't matter if the copied cell was empty. Only happened on some files.

What was causing it: When you copy a cell in Excel it draws those animated dashes around the copied cell (the "marching ants"). That animation is a loop that keeps redrawing constantly until you press Escape or paste.

If you have your display set to 125% scaling, Windows can't do clean math to scale graphics, so it has to do expensive interpolation on every single graphics call. Excel's marching ants fires a lot of those graphics calls per second, and the CPU has to work hard to do the calculations.

I found 3 ways to fix it:

1. Set display scaling to 100% - simplest fix, but text gets smaller
2. Disable the marching ants - in registry set HKCU\Control Panel\Desktop\WindowMetrics\MinAnimate to 0. No animation = no loop = no spike.
3. Tell Excel to handle its own scaling - right-click EXCEL.EXE → Properties → Compatibility → check "Override high DPI scaling behavior" → set "Scaling performed by" to Application. This is the cleanest fix - keeps 125% scaling and keeps marching ants, no spike.

Hope this helps someone.